TRIAL PATELLA COMPONENT KIT

Abstract

A kit for use in knee replacement surgery includes a trial patella component and a patella implant component. The trial patella component includes a backing plate having a backing surface for positioning against a patient's patella, and at least one sensor which can generate a signal corresponding to the compressive load applied to the trial patella component, and a first bearing plate which can be fitted to the backing plate and which, when fitted to the backing plate, provides a bearing surface for articulation against a bearing surface on the patient's femur. The patella implant component has a backing surface for fixation to a patient's patella instead of the backing surface of the trial patella component, and a bearing surface for articulation against a femoral bearing surface. The thickness of the patella implant component is greater than the thickness of the trial patella component when the first bearing plate is fitted to the backing plate, the difference between the said thicknesses being at least about 1 mm.

Claims

1. A kit for use in knee replacement surgery, which comprises: a trial patella component comprising: a backing plate having a backing surface for positioning against a patient's patella, and at least one sensor which can generate a signal corresponding to the compressive load applied to the trial patella component, and a first bearing plate which can be fitted to the backing plate and which, when fitted to the backing plate, provides a bearing surface for articulation against a bearing surface on the patient's femur, a patella implant component having a backing surface for fixation to a patient's patella instead of the backing surface of the trial patella component, and a bearing surface for articulation against a femoral bearing surface, in which the thickness of the patella implant component is greater than the thickness of the trial patella component when the first bearing plate is fitted to the backing plate, the difference between the said thicknesses being at least about 1 mm.

2. A kit as claimed in claim 1, in which the difference between the thickness of the patella implant component and the thickness of the trial patella component which is provided by the backing plate and the first bearing plate is at least about 3 mm.

3. A kit as claimed in claim 1, including a differential thickness bearing plate which can be fitted to the backing plate instead of the first bearing plate and which, when fitted to the backing plate, provides a bearing surface for articulation with a bearing surface on the patient's femur, in which the thickness of the differential thickness bearing plate is less than the thickness of the first bearing plate.

4. A kit as claimed in claim 1, including a differential inclination bearing plate which can be fitted to the backing plate instead of the first bearing plate and which, when fitted to the backing plate, provides a bearing surface for articulation against a bearing surface on the patient's femur, in which the difference in thickness between first and second opposite edges of the first bearing plate is different from the difference in thickness between corresponding first and second edges of the differential inclination bearing plate.

5. A kit as claimed in claim 1, which includes a first shim which can be fitted between the backing plate and the first bearing plate.

6. A kit as claimed in claim 5, which includes a second shim which can be fitted between the backing plate and the first bearing plate instead of or in addition to the first shim.

7. A kit as claimed in claim 6, in which the thickness of the second shim is different from the thickness of the first shim.

8. A kit as claimed in claim 6, in which the difference in thickness between the medial and lateral edges of the first shim is different from the difference in thickness between the medial and lateral edges of the second shim.

9. A kit as claimed in claim 1, in which the thickness of the trial patella component which is provided by the backing plate and the first bearing plate is not more than about 6 mm.

10. A kit as claimed in claim 1, in which the thickness of the patella implant component is at least about 7 mm.

11. A kit as claimed in claim 1, in which the trial patella component has a handle which extends outwardly from a patient's joint when the trial patella component is positioned with the bearing surface of the first bearing plate in contact with a bearing surface on the patient's femur.

12. A kit as claimed in claim 1, which includes a cutting guide having a support surface which can be engaged by a cutting instrument, and a mount which is configured to locate the cutting guide on a resected surface of the patella so that the cutting instrument is located relative to the resected surface of the patella when it engages the support surface to perform a secondary resection.

13. A kit as claimed in claim 12, in which the cutting guide includes a clamp for fastening the cutting guide to the patella with the mount located on a resected surface of the patella.

14. A kit as claimed in claim 12, which includes a cutting instrument having a cutting head for cutting the patella on a selected resection plane.

15. A kit as claimed in claim 1, which includes a patella implant trial component for temporary fixation to a patient's patella and for articulation against a femoral bearing surface, the thickness of the patella implant trial component corresponding generally to the thickness of the patella implant component.

16. A kit as claimed in claim 1, in which the backing plate has at least one pin which can penetrate a prepared surface of a patient's patella to locate the backing plate on the patella.

17. A kit as claimed in claim 1, in which the backing plate and the first bearing plate comprise co-operating means to removably fix the backing plate on the bearing plate during use.

18. A kit as claimed in claim 17 in which the surface of the first bearing plate which contacts the backing plate, and the surface of the backing plate which is contacted by the first bearing plate have mating spigot and socket features for locating the first bearing plate on the backing plate, optionally in which the spigot is provided on the backing plate, and/or in which the spigot is wider towards its free end than at its opposite root end.

19. A kit as claimed in claim 17, wherein the backing plate comprises one or more locating pins extending vertically from the surface of the backing plate that faces away from the patella in use, and the first bearing plate comprises corresponding bores for sliding over the one or more locating pins to locate the bearing plate or shim on the backing plate.

20. A kit as claimed in claim 17, wherein any additional bearing plates and/or shims also comprise complementary means to permit removable fixing to the backing plate.

21. A kit as claimed in claim 1, in which the backing plate has at least two sensors which are spaced apart along the superior-inferior axis and/or in which the backing plate has at least two sensors which are spaced apart along the medial-lateral axis.

Description

INTRODUCTION TO THE DRAWINGS

[0082] The invention is described below by way of example with reference to the accompanying drawings, in which:

[0083] FIG. 1 is a side elevation view of a trial patella component as can be incorporated in the kit of the invention.

[0084] FIG. 2 is an isometric view of a kit which includes a backing plate together with a plurality of bearing plates, with one of the bearing plates mounted on the backing plate to create a trial patella component as shown in FIG. 1.

[0085] FIG. 3 is an isometric view from below of a backing plate.

[0086] FIG. 4 is an exploded isometric view of a trial patella component as shown in FIG. 1.

[0087] FIG. 5 is a cross-section view of a trial patella component.

[0088] FIG. 6 is an isometric sectional view from one side of a trial patella component.

[0089] FIG. 7 is an isometric view of a trial patella component kit with a trial patella component in position in relation to a patella, viewed approximately along the medial-lateral axis.

[0090] FIG. 8 is an isometric view of another trial patella component kit viewed approximately along the superior-inferior axis.

[0091] FIG. 9 is an isometric exploded view of a trial patella component kit, similar to that shown in FIG. 7.

[0092] FIGS. 10 and 11 shows elevation views along the superior to inferior axis of sets of bearing plate components which can be used in a trial patella component.

[0093] FIG. 12 shows elevation views along the superior to inferior axis of a set of shims which can be used with bearing plate components such as those shown in FIGS. 10 and 11 in a trial patella component.

[0094] FIG. 13 shows elevation views along the superior to inferior axis of a set of shim and bearing plate component combinations.

[0095] FIG. 14 is an elevation view along the superior to inferior axis of a patella after an initial resection step and with an initial bearing plate in place.

[0096] FIG. 15 is another elevation view along the superior to inferior axis of the patella as shown in FIG. 14, with a different bearing plate assembly in place.

[0097] FIGS. 16 to 21 are graphs which show the variation in load that is sensed by lateral, superior and medial load sensors on a trial patella component during flexion, in which the y-axis units reflect the electrical output from the load sensors which is an indication of sensed load.

[0098] FIGS. 22(a) to (c) are views of a patella showing schematically different patella resection steps.

[0099] FIGS. 23(a) to (c) are schematic end views of a femur-patella combination which show how the position of the patella resection can be optimised by this invention.

DETAILED DESCRIPTION

[0100] Referring to the drawings, FIG. 1 shows a trial patella component 2 which comprises a backing plate 4 and a bearing plate 6. The backing plate 4 has two pins 8 which have pointed tips 10. The bearing plate 6 is located on the backing plate by means of spigot 12 (see FIG. 4) which is received in a socket. A spigot can be provided on the surface of the backing plate which is opposite to the surface on which the pins are provided, and a socket to receive the spigot can be provided on the bearing plate.

[0101] The bearing plate 6 has a bearing surface 14 which is contoured for articulation with a bearing surface on a patient's femur. The bearing surface on the femur can be the surface of the patient's natural femur. The bearing surface on the femur can be the surface of a primary femoral implant component which is to be replaced in a revision procedure (in which an already implanted knee joint prosthesis is replaced by a revision prosthesis, for example because of wear or loosening of one or more components of the already implanted prosthesis). The bearing surface on the femur can be the surface of a trial femoral component which is to be replaced by a femoral implant component.

[0102] The bearing plate 6 has three protrusions 16 on the surface which is opposite to the bearing surface 14, which are aligned with respective flex panels on the backing plate.

[0103] FIG. 2 shows components of a kit for use in knee replacement surgery which includes the trial patella component 2 shown in FIG. 1. The kit includes four bearing plates 6a, 6b, 6c, 6d in addition to the bearing plate 6 which is shown as part of the trial patella component shown in FIG. 1. The bearing plates differ from one another in terms of their thicknesses. The difference between the thicknesses of any two bearing plates whose thicknesses is closest is 1 mm. Each of the bearing plates has a bearing surface 14, and three protrusions (not shown in FIG. 2) on the surface which is opposite to the bearing surface.

[0104] FIG. 3 shows the backing plate 4 which has three flexible panels 20. Each of the flexible panels is connected to the main body of the backing plate along one edge 22. A slot 24 separates each panel from the main body of the backing plate along its other edges. A small hole 26 (which is shown as a through hole in the drawing but could be a blind hole which is open only on the surface of the backing plate which faces the bearing plate) is provided in each of the flexible panels. A handle 28 is formed from the same sheet of material as the backing plate, extending from the main body of the backing plate. The thickness of the backing plate is about 1 mm. The width of each of the flexible panels is about 6 mm. The length of each of the flexible panels is about 10 mm. Strain gauges are provided on the backing plate. A strain gauge is associated with each of the flexible panels to measure the strain in each panel as a result of load applied to the backing plate by the bearing plate. The incorporation of a strain gauge in association with a flexible panel of the type shown in FIG. 3 so that readable signals are generated reflecting the strain and therefore the applied load is known.

[0105] FIG. 4 is an exploded isometric view from above of the trial patella component 2, showing in particular the spigot 12 on the backing plate. It is wider at its free end 30 than at its base. A sleeve 32 of a resiliently deformable material fits over the spigot.

[0106] FIG. 5 is a cross-section view through the trial patella component shown in FIG. 4, showing the spigot 12 with the sleeve 32 fitted over it, received in a socket 34 in the bearing plate 6. The size of the socket at its open end is slightly smaller than the size of the spigot at its free end when the sleeve is fitted over the spigot, so that the sleeve has to be compressed slightly in order for the spigot to be received in the socket. The use of a resiliently deformable material for the sleeve means that it is able to expand towards the shape and configuration that it had before being compressed, so that separation of the backing plate and the bearing plate is resisted.

[0107] FIG. 6 is a side elevation of the trial patella component shown in FIG. 4, showing how one of the protrusions 16 on the bearing component engages one of the small holes 26 that is provided in the flexible panels 20 in the backing plate.

[0108] The kit includes a patella implant component, and optionally also a patella implant trial component. The designs of these components will closely follow those of existing components. The kit of the invention provides the trial patella component which enables appropriate preparation of a patient's patella to allow use of patella components of existing designs.

[0109] The invention has been assessed using bearing plate components of a trial patella component. The bearing plate shown in at least FIGS. 1 to 6 is and has been made by 3D printing. The properties of the material are as follows:

TABLE-US-00001 PROPERTY TEST METHOD VALUE Colour White Sintered density ASTM D792 0.93 g .Math. cm.sup.3 Water absorption, 20EC, DIN EN ISO 62 0.5 0.2% 50% Relative Humidity Water absorption, 24 h 2.0 0.3% in boiling water E-Module (x-y plane) DIN EN ISO 527, 2000 200 MPa E-Module (z plane) test speed 1900 200 MPa Tensile strength (x-y) 10 mm .Math. min.sup.1 50 4 MPa Tensile strength (z) 42 5 MPa Elongation at break 11% 4%.sup. (x-y) Elongation at break (z) 4% 2%.sup. Vicat softening point ISO 306 163EC (50 C. .Math. h.sup.1 50N)

[0110] The invention has been assessed using various backing plate components. The backing plate shown in FIGS. 1 to 6 is made from a Ti6Al4V alloy and is subjected to a heat treatment to optimise its physical properties. The properties of the material are as follows:

TABLE-US-00002 Tensile Yield Strength Elongation Strength MPa 0.2% MPa % Hardness >930 MPa >860 MPa >10% 33 +/ 2 HBW

[0111] The embodiments shown in FIGS. 1 to 6 include a sleeve of a resiliently deformable material fitted over the spigot on the backing plate component, where the sleeve has been made from a silicone rubber by screen printing. The rubber has a Shore A hardness of 65 (measured according to ASTM D-2240), with a tensile tear strength of between 10 and 12 kgf.cm.sup.1 (measured according to ASTM D-624), an elongation to break of between 120 and 170% (measured according to ASTM D-412) and a tensile strength of between 4.0 and 4.5 MPa (measured according to ASTM D-412).

[0112] It is estimated that the maximum load to which a patella is subjected throughout the range of motion of a knee in a typical patient is about 200 N. The backing plate is able to measure loads in excess of this, with deformation of the flexible panels in the backing plate in response to the compressive loads applied to the trial patella component through the bearing plate.

[0113] Suitable strain gauges for use in the backing plate are capable of measuring strain of up to 10% when a load of up to 200 N is applied to the flexible panels. Signals from the strain gauges can be processed using an appropriate analog to digital converter and an appropriate data processor. An analog to digital converter and a data processor from Friends-of-Fritzing eV have been used in testing.

[0114] FIG. 7 shows a trial patella component 50 in contact with a patella 54 which has been resected to create a planar resection surface which faces posteriorly. The resection can be performed using a conventional patella resection guide. The depth of the resection is about 5 mm.

[0115] The trial patella component 50 comprises a backing plate 56 and a bearing plate 58. The backing plate is made from stainless steel and has a thickness of 1 mm. The bearing plate is formed from glass fibre reinforced nylon and has a planar surface for contacting the backing plate and an opposite bearing surface 60. The bearing surface has a ridge 62 which extends approximately in line with the patient's superior-inferior axis when the trial component is positioned in the patient's joint, extending generally parallel with the medial edge 64 of the plate (and also the lateral edge which is not visible in the drawing). The ridge defines a saddle point 66 at approximately its midpoint. The shape of the bearing surface of the bearing plate shown in FIG. 7 is similar to the shape of the bearing surface of a native patella and is referred to as an anatomic bearing surface.

[0116] The backing plate has pins on the surface which faces towards the patella, and which can penetrate the surface of the patella to fix the trial patella component to the patella by preventing it from sliding on the patella. The pins are not visible in the drawing. The backing plate also incorporates force sensors.

[0117] The patella kit shown in FIG. 7 includes alternative bearing plates 68, 70, which differ from one another and from the bearing plate which is positioned on the backing plate in terms of their thickness, the difference in thickness between each plate being 1 mm. Each of the bearing plates in the kit shown in FIG. 7 has an anatomic bearing surface.

[0118] The kit also includes a shim 72 which can be positioned between the backing plate and a selected one of the bearing plates. The shim is formed from glass reinforced nylon, and is 3D printed. The shim has a thickness of 1 mm.

[0119] Each of the bearing plates and the shim has a pair of spaced apart bores 74 extending through it for receiving locating pins which extend from the backing plate. The pins are a sliding fit in the bores in the selected bearing plate, and the shim if used, and serve to locate the selected bearing plate and shim on the backing plate.

[0120] FIG. 8 shows a trial patella component 80 in contact with a patella 84 which has been resected to create a planar resection surface which faces posteriorly. The resection can be performed using a conventional patella resection guide which can be set to an appropriate resection depth.

[0121] The trial patella component 80 comprises a backing plate 86 and a bearing plate 88. The backing plate and the bearing plate are formed from the same materials as the corresponding components in FIG. 7. The bearing plate has a planar surface for contacting the backing plate and an opposite bearing surface 90. The thickness of the bearing plate is greatest at a point that is spaced from all of its edges (medial, lateral, superior, inferior), for example approximately central point. The shape of such a bearing surface is referred to as a domed bearing surface.

[0122] The patella kit shown in FIG. 8 includes alternative bearing plates 92, 94, which differ from one another and from the bearing plate which is positioned on the backing plate in terms of difference in thickness in each of the plates between the medial and lateral edges. It can be seen in the drawing that the thickness of the bearing plates 92, 94 is greater at the edge 96 which is remote from the handle 97 than at the edge 98 which is adjacent to the handle. Each of the bearing plates 88, 92, 94 has a thickness of 0.1 mm at the thinnest edge and a thickness at the opposite edge of 1 mm, 2 mm and 3 mm respectively. The bearing plates can be rotated so that the thicker edge can be positioned medial or lateral.

[0123] The kit also includes first and second shims 100, 102 which can be positioned between the backing plate and a selected one of the bearing plates. The shims differ from one another in terms of their thicknesses. In a particular embodiment, each of the shims has a constant thickness across its area, the thicknesses being 1 mm, 2 mm and 3 mm respectively. Alternatively, the thickness of a shim might be greater at one edge compared with an opposite edge so that its thickness is tapered. Such a shim can be described as wedge-shaped. In the embodiment of FIG. 8, the shims have thicknesses at the thickest edge of 1 mm, 2 mm and 3 mm respectively. The shims are formed from glass reinforced nylon.

[0124] Each of the bearing plates and the shim has a pair of spaced apart bores 104 extending through it for receiving locating pins which extend from the backing plate. The pins are a sliding fit in the bores in the selected bearing plate, and the shim if used, and serve to locate the selected bearing plate and shim on the backing plate.

[0125] FIG. 9 shows a trial patella component positioned in relation to the posterior face of a patella 120. The trial patella component has a backing plate 122. The backing plate has a sensor lobe 124 which can be positioned on a resected surface of the patella and a handle lobe 126. The backing plate is formed from a rigid polymeric material such as a polyamide or a polycarbonate or a polyester, and has a thickness of 0.5 mm. The backing plate has a pair of cylindrical pins extending from its upper face. The pins are spaced apart along the superior-inferior axis. A sensor sheet 128 is positioned on the sensor lobe of the backing plate. The sensor sheet is a FlexiForce HT201 sensor, available from Tekscan Inc. and having a thickness of 0.3 mm, but any conventional sensor could be used. The sensor sheet carries superior, medial, lateral and inferior load sensors 130, 132, 134, 136. The sensor sheet has embedded in it conductors for supplying electrical power to the load sensors and conductors for signals which are generated by the load sensors. The sensor sheet has a pair of holes 138 formed in it in which the pins on the backing plate are a close fit to locate the sensor sheet on the backing plate.

[0126] The trial patella component includes a handle 140 which is fastened to the handle lobe 126 of the backing plate. The handle is formed from glass reinforced nylon and includes a chamber 142 having a port 144 for a cable. The chamber can contain connectors between conductors in a cable and the conductors in the sensor sheet. The handle can be gripped by a user of the component. The handle lobe might be truncated or omitted when it is desirable to make the component more compact. A handle might extend transversely without a limb which extends superiorly or inferiorly.

[0127] The trial patella component kit includes three bearing plates 150, 152, 154. The bearing face 156 of each of the bearing plates is an anatomic bearing surface, similar to that described above with reference to FIG. 7, and the bearing plates are formed from the same materials as the bearing plates of FIG. 7.

[0128] The kit also includes three shims 158, 160, 162. The shims that are shown in FIG. 9 have a constant thickness. Alternatively, one or more of the shims might be thicker at one edge than at its opposite edge so that it is wedge-shaped. The shims are formed from the same materials as the shims of FIG. 8.

[0129] Each of the bearing plates has a pair of holes 164 formed in it in which the pins on the backing plate are a close fit to locate a selected one of the bearing plates on the backing plate. Each of the shims has a pair of holes 166 formed in it in which the pins on the backing plate are a close fit. One or more shims can be located on the backing plate, between it and a selected one of the bearing plates.

[0130] FIG. 10 is a view of the superiorly facing edges of three bearing plates 170, 172, 174. Each of the bearing plates has an anatomic bearing surface. The bearing plates differ from one another in their thicknesses. The difference in thickness between the pairs of plates whose thicknesses are closest is 1 mm. Specifically, the thickness of each bearing plate (excluding the anatomic bearing surface) is 1 mm, 2 mm, and 3 mm.

[0131] Each of the bearing plates has pads 176 on its lower surface (opposite to its bearing surface). The positions of the pads correspond to the positions of the load sensors 130, 132, 134, 136 on the sensor sheet 128 so that the pads on a selected one of the bearing plates contact the load sensors when the plate is positioned on the sensor sheet.

[0132] FIG. 11 shows bearing plates 182, 184, 186 also viewed along the superior-inferior axis. Each of the bearing plates has a medial edge 188 and a lateral edge 190. The three bearing plates shown in FIG. 11 have the same thickness at their medial edges. The thicknesses of the three bearing plates shown in FIG. 11 differ at their lateral edges, the difference in thickness being 0.5 mm between each plate. The bearing plates have pads 192 on their lower surfaces.

[0133] FIG. 12 is a view of the superiorly facing edges of three shims 200, 202, 204. The shims differ from one another in their thicknesses. The difference in thickness between the pairs of shims whose thicknesses are closest is 1 mm. Specifically, the first shim 200 has a thickness of 1 mm, the second shim 202 has a thickness of 2 mm and the third shim 204 has a thickness of 3 mm. Each of the shims that is shown in FIG. 12 has a constant thickness across its principal surface. Alternatively, one or more of the shims might be thicker at one edge than at its opposite edge so that it is wedge-shaped.

[0134] Each of the shims has pads 206 on one of its principal surfaces. The positions of the pads correspond to the positions of the load sensors 130, 132, 134, 136 on the sensor sheet 128 so that the pads on a selected one of the shims contact the load sensors when the shim is positioned on the sensor sheet.

[0135] FIG. 13 is a view of the superiorly facing edges of three combinations of a backing plate and shim. Each of the combinations uses the thinnest 170 of the bearing plates shown in FIG. 10. The bearing plate is shown with no shim, and with the thinner two 200, 202 of the three shims shown in FIG. 12, respectively.

FIG. 14 shows a patella 220 which has been resected to remove its posterior bearing surface. The resected bone material 222 is shown in FIG. 14.

[0136] A trial patella component similar to the one shown in FIG. 8 is positioned with the backing plate 86 in contact with the resected patella and a domed bearing plate 88 positioned with the pins on the backing plate received in the bores on the bearing plate. A sensor sheet is located on the backing plate. The bearing plate has pads 224 on the surface which faces towards the backing plate. The bearing plate is positioned so that the pads are in contact with the load sensors on the sensor sheet.

[0137] The thickness of the trial patella component that is fitted to the patella immediately after the initial resection corresponds to the depth of the resection. Flexion of the knee with the trial patella component acting against the patient's native femur enables data to be obtained from the load sensors for loads applied to the patella which approximate to the loads that are likely to be experienced in the patient's native patella.

[0138] FIG. 15 shows the patella 220 and the trial patella component with the backing plate 86 in contact with the patella. The trial patella in the configuration shown in FIG. 16 has a shim 226 and a bearing plate 228 whose thickness at its lateral edge 230 is 3 mm and the thickness at its medial edge 232 is 1 mm.

[0139] The kit provided by the invention can be used in a surgical procedure as follows:

[0140] A first step in the procedure which involves the kit of the invention is a conservative resection of the patella. The trial patella component is then fitted to the resected patella with the backing plate pressed against the surface of the patella which is exposed by the resection and pins on the backing plate penetrating into the resected patella so as to fix the backing plate to the patella, by preventing it from sliding on the patella.

[0141] The thickness of the trial patella component, made up of the bearing plate and the backing plate, corresponds to the thickness of bone that is removed in the conservative resection.

[0142] The joint is then flexed with the bearing surface provided by the bearing plate of the trial patella component acting against the bearing surface of the patient's natural femur. The load to which the patella is subjected can be measured throughout the range of motion of the joint using the trial patella component.

[0143] The surgical procedure then involves locating the planes for resection of the patient's femur and tibia using conventional instrumentation, and fitting trial femoral and tibial components to the resected bones.

[0144] The joint is flexed with the trial femoral and tibial components in place, and with the bearing surface provided by the bearing plate of the trial patella component acting against the bearing surface provided by the trial femoral component. The load to which the patella is subjected can be measured throughout the range of motion of the joint using the trial patella component. The load to which the patella is subjected can be increased by substitution of the bearing plate component of the trial patella component by a bearing plate having a greater thickness. The load to which the patella is subjected can be reduced by substitution of the bearing plate component of the trial patella component by a bearing plate having a smaller thickness or having different thickness different between opposite edges. The substitution of the bearing plate component has the aim to achieve a desirable tension in the patient's quadriceps muscle throughout the range of motion of the joint, frequently as similar as possible to that in the quadriceps muscle at the start of the procedure. Features which it can be desirable to match include the absolute load sensed by each sensor, the difference in loads between pairs of sensors (for example between medial and lateral sensors), and the flexion angles at which peak load is sensed by one or more of the sensors. Bearing plates having thicknesses from 2 mm to 7 mm, and with differences in thickness between opposite edges of 1 mm or 2 mm, might be included in the kit for this purpose.

[0145] The total thickness of the trial patella component, with the selected bearing plate, allows the surgeon to determine the amount of patella tissue that should be removed in a second resection which creates the resection surface which a patella implant component can be fitted to, with the intention of ensuring that the total effective thickness of the patella with the trial patella component in place is the same as the total effective thickness of the patella with the patella implant component in place, taking into account the greater thickness of the patella implant component compared with the trial patella component with its selected bearing plate.

[0146] FIGS. 16 to 21 show the variation in load sensed by lateral, superior and medial sensors in a trial patella component of the kind described above with reference to FIG. 8. The data have been obtained using a sawbones model, and using components from a knee replacement system as sold by DePuy International Limited under the trade mark Attune.

[0147] FIG. 16 shows the variation in load using a trial patella component after performing a conservative resection on the patella, as described above with reference to FIG. 14. The trial patella component articulated against the bearing surface provided by the femur component of the sawbones model.

[0148] The femur and tibia were then prepared for receiving femoral and tibial trials respectively by performing (a) a 9 mm distal femoral cut with a 5 degree varus valgus adjustment, (b) a 9 mm tibial resection with a 5 degree anterior slope, and (c) posterior femoral cuts using a size 5 cutting block, set in the neutral position.

[0149] The model knee was then flexed with the trial patella component in place, fitted with constant thickness bearing plates, of the type shown in FIG. 10. The variation in load during flexion is shown in FIGS. 17 (5 mm bearing plate) and 18 (7 mm bearing plate).

[0150] The model knee was then flexed with bearing plates whose thicknesses differ between the medial and lateral edges. The bearing plates were of the type shown in FIG. 11, with shims as shown in FIG. 12. The variation in load during flexion is shown in FIGS. 19 (bearing plate thickness: 5 mm medial; 6 mm lateral), 20 (bearing plate thickness: 7 mm medial; 8 mm lateral) and 21 (bearing plate thickness: 6 mm medial; 7 mm lateral).

[0151] It can be seen that there are similarities between the load variation graphs in FIGS. 16 and 21 in the maximum loads sensed by the three sensors and the relationship between the flexion angles at which those maximum loads are sensed.

[0152] The nature of the two step resection that is intended when using the kit of the invention can be understood with reference to FIG. 22. FIG. 22(a) shows a patella 350 after the initial shallow resection step to remove a portion 352 of the patella having a depth of about 4 to 5 mm. FIG. 22(b) shows the patella after the second resection step to remove a further portion 354 of the patella and fitting of an implant component 356 to the patella. In contrast, FIG. 22(c) shows the result of a conventional procedure in which a single resection step is performed on a patella 360 to remove a portion of the patella 362 prior to fitting the implant component 356. The depth of the portion 362 of the patella that is removed using the conventional procedure is more than the sum of the depths of the portions 352, 354 of the patella that are removed when the kit of the present invention is used. Furthermore the present invention enables the orientation of the final resection plane of the patella to be selected as can be seen from the shape of the further resection portion 354 shown in FIG. 22(b).

[0153] FIG. 23 depicts schematically views of three femur-patella combinations, showing the distal face of the femur 300 and the patella 302 in contact with the femur in the position that it has when the knee is extended.

[0154] FIG. 23(b) shows a natural knee in which the anterior face 304 of the patella 302 defines an anterior guide line 310 and the posterior femoral condyles 306 define a posterior guide line 312.

[0155] FIG. 23(c) shows a knee in which a femoral implant component 314 has been fitted to the femur 300 and a patella implant component 316 has been fitted to the patella 302. It can be seen that the bearing surface provided by the femoral implant component varies from the bearing surface provided in the natural knee in that the posterior condyle 306 is located posteriorly of the posterior guide line 312. This can result from the steps taken to position the tibial and femoral components of the prosthesis and is a normal occurrence.

[0156] Following conventional knee replacement surgical procedures, the thickness of the patella implant component will be chosen so that it matches the thickness of the portion of the patella that is resected. The consequence of this, as shown in FIG. 3(c) is that the anterior face 304 of the patella 302 is located posteriorly of the anterior guide line 310, and the tension in soft tissue connected to the patella will be less than in the natural knee.

[0157] FIG. 23(a) shows that a smaller final resection of the patella, as described above with reference to FIGS. 22(a) and (b), can restore the position of the patella relative to the anterior guide line and therefore restore the tension in soft tissue connected to the patella so that it is closer to that in the natural knee. As discussed herein, the invention facilitates identification of the appropriate plane for resection of the patella to achieve this result. This can provide better balance in the patient's quadriceps mechanism which can improve the long term outcome of the knee replacement procedure.